1. Field of the Invention
The present invention relates to spring set configurations for microelectromechanical systems (MEMS) devices, and more particularly to spring set configurations for MEMS gyros.
2. Description of Related Art
A variety of gyroscope devices are known for providing navigational guidance such as in aerospace applications. Microelectromechanical systems gyroscopes (hereinafter MEMS gyros) are known for their compact size and relatively low cost of manufacture. In addition to aerospace applications, the small size and low cost of MEMS gyros lends them well to a variety of other applications including motion sensing for image stabilization and input devices, for example.
Gyros operate by moving a mass. When the mass is located in a rotating reference frame, for example, it will be subjected to a Coriolis force calculated by the formula:Fc=2mv×Ω, where Fc is the Coriolis force, m is the mass of the moving body, v is the velocity vector for the rotating body, and Ω is the angular velocity vector for the rotating body. Conventional spinning mass gyros generate large Coriolis forces by spinning at high velocities. MEMS gyros typically do not have bearings on which they can continually spin. Instead, MEMS microstructures create motion by vibrating a mass. When the mass is vibrated at its natural frequency, large amplitudes can be achieved with minimal excitation. When this driven mode is excited in a rotating reference frame, the resulting Coriolis force will be perpendicular to the driven mode direction due to the cross product of the velocity and angular velocity vectors in the formula above. This motion in the perpendicular direction is what is sensed to determine rotational rate of the reference frame. The mass is driven at its drive resonance frequency, thus the sensed motion will also vibrate at the same frequency but in an orthogonal direction. If the microstructure is designed such that the sensed motion natural frequency is close to the driven frequency, the resulting motion will be gained dynamically. The amount of this dynamic gain (Q) can be described byQ=ωdrive/Δω,where ωdrive is the driven frequency and Δω is the difference in frequency between the driven and sensed modes. The motion can be sensed and driven capacitively.
Δω is a key parameter for resonant gyros. The smaller the value of Δω, the greater the gain. But this increased gain comes at a cost in the form of decreased bandwidth of rotation that can be detected. Since Δω is the difference between the sense and drive frequencies, small variations in these nominal frequencies can cause relatively large fluctuations in Δω. Therefore, consideration must be made to ensure that process variations affect both sense and drive frequencies in the same or similar amounts.
A typical example of such a MEMS gyro includes a microstructure that is driven in-plane, oscillating about the z-axis. If the reference frame is rotated about the x or y-axis, Coriolis motion will be produced about the y or x-axis, respectively. Such a gyro can be optimized to sense reference frame rotations about only one axis, for example the x-axis. This is achieved by placing the majority of the mass close to the x-axis and as far from the y-axis as possible. This minimizes the moment of inertia about the x-axis and maximizes the moment of inertia about the y-axis. This also makes the lowest resonant mode the desired y-axis rotation. Electrodes with opposite polarity (high and low bias) are placed under the microstructure on either side of the y-axis so when the capacitance is changed a current is generated that can be converted to a voltage with a charge amplifier. If the microstructure were to rotate about the x-axis, there would be no net current generated because both capacitances would change equally.
In ideal operation, a point “p” on the end of the drive motor will move back and forth in the y-direction as the gyro is driven. If the reference frame is not rotating, the point p will only move in the y-direction and will not move in the z-direction. When the reference frame rotates about the x-axis, a Coriolis force is generated proportional to the velocity in the z-direction according to the formula for calculating Fc above. The motion generated is proportional to the Coriolis force.
Quadrature motion is generated when point p moves in the z-direction in its driven mode. This results in a driven motion that is 90 degrees out of phase with the rate rotation sense signal. The signal from the gyro is sinusoidal at the driven frequency with phase components from both the desired rotation rate signal and from the quadrature. The two phases of the signal are decoupled by a demodulation circuit. The demodulation circuit provides an output signal that includes an average amplitude of the in-phase rate signal as well as the out of phase quadrature signal. The in-phase rate signal is the desired output signal the gyro is designed to sense.
When an unwanted quadrature signal is too large, it can cause the charge amplifier to clip and any information of the desired rate signal is clipped along with it. Electrical mitigation circuits have been utilized to reduce this effect of quadrature error on the desired rate signal. Typical quadrature error mitigation circuits work by applying both high and low bias voltages on compensation electrodes. This generates a current with the in-plane driven motion having the same phase as the unwanted quadrature. By applying a compensation voltage bias, the quadrature mitigation circuit can minimize the unwanted quadrature signal. The amount of compensation voltage bias needed to minimize the unwanted quadrature signal is an indicator of how far the microstructure is tipping out of plane. Design improvement can be measured by how much compensation voltage is reduced. The quadrature error mitigation circuit is limited by the available voltage. It is not uncommon for quadrature error to be so large that it cannot be corrected with a quadrature error mitigation circuit. Some gyro designs have larger compensation electrodes so that more current can be generated and larger quadrature error signals can be minimized. Larger compensation voltages may result in unacceptable noise levels in the device. Minimizing unwanted quadrature by electrical means can make the microstructure useable, but high level performance characteristics such as Allan variance and temperature sensitivity may be compromised since they have been correlated to compensation voltage levels, probably due to the microstructure moving out of plane.
A primary cause of quadrature error is the etch angle variation in the microstructure components, and particularly in the springs. An ideal orthogonal spring will move in the direction it is forced, but when there is an etch angle producing an angled neutral axis (for instance a parallelogram cross-section), i.e., a tilt of some degree, the spring will also move out of plane to the forcing direction.
Deep reactive-ion etching (DRIE) tools are state of the art tools typically used to construct MEMS devices. DRIE tools use etch chemistry in a directional plasma to etch silicon vertically. These tools can have a radial center-to-edge variation in etched angle due to edge effects of the plasma. The orientation of the etch angle can be dependent on where the die is located on the wafer with the straightest edges (least tilt) being produceable only in a correlated portion of the wafer.
The driven mode is affected by both the orientation and magnitude of the etch angle. When the etch angle direction is orthogonal to the spring direction, it maximizes the out of plane component of motion. When the etch angle direction is the same as the spring direction, there is little effect. When an etch angle is present on the gyro in the y-direction, the out of plane component is generated by the spring component in the x-direction, and this causes a rotation about the y-axis. The opposite is true for etch angles in the y-axis in that they generate an out of plane motion about the x-axis. Only out of plane motions about the y-axis produce a signal, and thus quadrature, as described above.
The magnitude of quadrature displacement is affected by the differences in natural frequencies of the quadrature mode and the driven mode. In the same way that the gyro output is gained dynamically due to the sense mode being close to the driven mode, the quadrature motion is also gained. X-direction etch angles cause out of plane motion about the x-axis. Since this mode is far from the driven frequency, there is not much dynamic gain present. This motion does not generate an electrical current since the out of plane capacitors change equally. However, y-direction etch angles cause out of plane motion about the y-axis. This mode is intentionally close to driven mode because it is needed to amplify the desired Coriolis motion. Consequently, the quadrature motion about the y-axis is amplified by its dynamic gain (Q), and its motion produces an electrical quadrature signal.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for spring set configurations on MEMS devices and particularly on MEMS gyros that allow for reduced sensitivity to etch angle errors (or inaccuracies) due to processing variations. There also remains a need in the art for such MEMS devices and MEMS gyros that are easy to make and use. The present invention provides a solution for these problems.